An optical disc drive is provided with a lens-actuator that keeps a laser spot in focus on an information layer of an optical disk and tracks the spot on the track on the optical disk for scanning the optical disk. This lens-actuator usually comprises a focus actuator and a tracking actuator, also called radial actuator, which are controlled by a focus actuator controller with a focus control bandwidth and a tracking actuator controller with a tracking control bandwidth. Each of these controllers cooperates with an optical spot position detector that measures a defocus and a de-tracking of the laser spot and controls the actuator to position the laser spot relative to the information layer and relative to the track.
For higher scanning speeds, requirements as to the performance of the actuators and the controllers become increasingly difficult to meet. E.g., a higher control bandwidth and/or a higher sensitivity may be required for focussing and tracking at high speeds. Moreover, optical disks show large differences in disk quality, e.g. in disk warp, eccentricity and local track acceleration errors. As a result, some disks are relatively easy to focus and track even with a moderate bandwidth or at high speeds, whereas other disks are hard to focus and track and would require a high bandwidth or a high sensitivity especially at high speeds.
However, there are several potential problems associated with operation of the actuator for focussing and tracking at high speeds.
E.g., due to dynamical characteristics of the actuator, the control loops are limited in bandwidth to achievable practical bandwidths up to about 8 kHz, which is generally too low for Blue-Ray disks at speeds of 4× and higher.
Even if the servo bandwidth is sufficient, it may happen that the actuator must make large excursions at high frequencies, e.g. to compensate for disk warp or eccentricity at high rotational speeds. Such disturbances can be corrected e.g. using feed-forward control using a memory loop, so that a high loop bandwidth is not required. However, the dissipation in the coils of the actuator may become unacceptable under such conditions.
In order to correct for high-frequency disturbances, and especially local acceleration errors in the axial or radial direction, the actuator must be able to provide sufficient acceleration. If this is not the case, the output of the servo driver may saturate at a maximum available voltage, and the residual error becomes too high, with the risk that even a focus loss or track loss might occur. The achievable acceleration at a maximum available voltage may also be referred to as the sensitivity of the actuator.
In a practical situation one or more of these problems may occur during reading or writing of a disk at a high speed. However, very often no problems occur at all. Whether a problem occurs or not depends very much on the characteristics of the disk at hand. Although optical disks are standardized with a plurality of disk standards, and each disk standard specifies maximum values for disk warp, eccentricity and acceleration errors, this does not mean that these maximum values will occur on each individual disk. In fact, the worst-case conditions described in the standard are often based on unjustified concerns for low yield, and agreed upon long before any real production data is available. The result is that the error values found in practice are often much lower than allowed by the disk standard.